There has been a semiconductor laser having a GaAs contact layer to which Zn is added (see, for example, Jurgen Sebastian et al., “High-Power 810-nm GaAsP-AlGaAs Diode Lasers with Narrow Beam Divergence”, IEEE JOURNAL ON SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 7, NO. 2, March/April 2001, P. 334-339).
An example of a laminated structure of a semiconductor laser having a GaAs contact layer to which Zn is added will be described with reference to FIG. 5.
As shown in FIG. 5, a n-type GaAs buffer layer 22 to which Si is added as a n-type impurity is formed on a n-type GaAs substrate 21. A n-type AlGaInP clad layer 23 to which Si is added is formed on the n-type GaAs buffer layer 22. A quantum-well structure formed of a non-doped InGaP guide layer 24, a non-doped GaAsP active layer 25 and a non-doped InGaP guide layer 26 is formed on the n-type AlGaInP clad layer 23. A p-type AlGaInP clad layer 27 to which Zn is added as a p-type impurity, a p-type InGaP-BDR (band discontinuity reduction) layer 28 to which Zn is added and a p-type GaAs contact layer 29 to which Zn is added are successively formed on the quantum-well structure. The film thickness and carrier concentration of each layer are shown in FIG. 5.
As a method of measuring the emission wavelength of a active layer of semiconductor laser, photoluminescence (hereinafter referred to as “PL”) measurement is known.
In ordinary cases, however, the optical bandgap of a contact layer of a semiconductor laser is smaller than the optical bandgap of an active layer. Also in the above-described semiconductor laser, the optical bandgap of the p-type GaAs contact layer 29 is smaller than the optical bandgap of the GaAsP active layer 25. Therefore, if a PL measurement is carried out on the semiconductor laser in the normal state, light is absorbed by the contact layer 29, so that the active layer 25 cannot be photoexcited. Therefore, removal of the contact layer 29 is required to enable photoexcitation of the active layer 25 in PL measurement. That is, a destructive test is required for measurement of the emission wavelength of the active layer.
Thus, PL measurements on the conventional semiconductor lasers have been destructive tests, and PL measurements cannot be made on active layers in products. PL measurements have therefore been made on active layers formed on test wafers. However, the results of PL measurements on active layers on test wafers are not always the same as the PL wavelengths of the corresponding active layers in products. For this reason, defective/nondefective determination with respect to the emission wavelength of a product cannot be made before the completion of an energization test on the product. In some case, the yield of a product is reduced due to this hindrance.